1. Field of the Invention
In at least one aspect, the present invention relates to the synthesis of coordination polymers formed from metal ions or clusters and organic ligands.
2. Background Art
The design, synthesis, characterization, and application of porous materials has broadly attracted both scientists and engineers due to the need to understand and control reactions or processes that occur in nanometer-sized spaces. This has a profound commercial impact because porous solids are widely used as sorbents in important applications such as molecule storage, separation and heterogeneous catalysis.
Historically, zeolites have played a central role in such fields. In contrast to structurally ambiguous porous materials such as activated carbons and glasses, zeolites are crystalline materials that have a very narrow pore size distribution. Zeolites, both natural (i.e. aluminosilicates) and synthetic (i.e. aluminophosphates) are important examples of microporous crystalline materials that consist of interconnected tunnels or cages resulting in cavities and pores that are uniform in size for a specific zeolite and thus can be utilized as molecular sieves where molecules small enough to fit into the pores are adsorbed but larger ones are not. The largest pore diameter in a zeolitic material is limited to about 13 Å. Using surfactants as structure directing agents or templates, mesoporous silicate/aluminosilicate materials have been synthesized with pores sizes that are tunable in the range of about 20 to 100 Å. Such materials do not display pores with size uniformity approaching natural zeolites and display non-uniform functionality on their inner surfaces.
Progress in the area of hybrid inorganic-organic materials has lead to the development of coordination compounds that now extend in 1-, 2-, or 3-dimensions. The term coordination polymer has been used to describe such compounds in which the backbones or frameworks of these materials are made by connecting metal ions or metal clusters (referred to as vertices of the framework) with organic ligands that when bridging multiple metal centers are commonly referred to as linkers or links. Other terms used to refer to coordination polymers include metal-organic frameworks (“MOFs”), porous coordination polymers (“PCPs”), nanoporous coordination frameworks, hybrid porous solids, and metallo-organic polymers. Rapid advances in the field have revealed that coordination polymers can be synthesized with porosity thereby constituting a new class of materials that are crystalline molecular sieves. The atomic structure of many coordination polymers can be determined by X-ray crystallography, thus the dimensions of the pores or channels can be determined with excellent certainty. The internal surface areas of some porous coordination polymers are significantly greater than other porous materials. The pore sizes/shapes are highly tunable, and larger pore sizes can be synthesized when compared to known zeolites. Functionalization of the backbones or frameworks in these materials can be achieved by starting the synthesis with organic linkers with functional groups already installed or by post synthesis modification.
Mesoporous coordination polymers (pore size=2-50 nm) are considerably more rare than microporous coordination polymers (pore size <2 nm) in part because as the size of the organic linker increases, the structural integrity of the resulting coordination polymer may decrease. This can result in a collapse of the pore structure and a loss of crystallinity once the guest molecules (typically the solvent of synthesis and synthetic components or byproducts) are removed. Alternatively, interpenetration or catenation is sometimes observed which limits the formation of mesopores through partial pore occupation. To date few examples of crystalline mesoporous coordination polymers have been reported, and those that have, may be categorized according to the geometry of their mesopores. The first type have 1D mesoporous channels. In these materials porosity is derived mainly from the contribution of well-defined mesopores and these do not exhibit the outstanding surface areas of their microporous counterparts. The second type of crystalline mesoporous coordination polymers exhibit mesoporous behavior due to a network of cages found throughout the structure. These mesoporous cages are often restricted by small apertures or windows that prohibit very large molecules from accessing the space inside.
Against this prior art background, there is still a desire for novel multidentate ligands that are useful for constructing porous materials.